1. Field of the Invention
The present invention relates to a bonding material applicable to unfired ceramic bodies, made of unfired ceramic materials, to each other and a method of manufacturing a ceramic bonded body using such a bonding material.
2. Description of the Related Art
In the related art, a ceramic bonded body, composed of ceramic bodies bonded to each other, has heretofore been widely used in various technical fields. In particular, the ceramic bonded body takes the form of a stack-type gas sensing element used for detecting a specified gas concentration of, for instance, measuring gases.
The stack-type gas sensing element is comprised of a ceramic body containing a solid electrolyte, and a ceramic body in which a measuring gas chamber is defined (see Japanese Patent Application Publication Nos. 2004-165274, 2006-173240, 2006-30165 and 2004-271515),
The stack-type gas sensing element can be manufactured in a manner as described below.
That is, first, ceramic powder, a binder, a plasticizer and a solvent are mixed to provide a mixture, which is then shaped to form unfired sheets each having a surface on which an electrically conductive paste is printed for forming an electrode pattern or a heating pattern or the like. Subsequently, a plurality of unfired ceramic sheets are stacked into a stack body, which is thermally compression bonded. The resulting stack body is then fired, thereby producing a gas sensing element composed of the ceramic sheets of a stacked structure.
An recent years, the stack-type gas sensing element has been complicated in structure, resulting in the occurrence of various issues in manufacturing the stack-type gas sensing element. In particular, the presence of a cavity portion through which gases are admitted to an internal part of the sensing element causes a serious problem on production of the sensing element.
That is as shown in FIG. 7A, in order to form a cavity in the sensing element, the sensing element 100 includes an unfired ceramic body 91, having a surfaces formed with a concaved portion 90 serving as a cavity portion, and an unfired ceramic body 92. The unfired ceramic body 92 is fixedly secured to the unfired ceramic body A 91 by thermal compression bonding. When this takes place, due to the thermal compression bonding, the cavity portion 90 is likely to be deformed. As a result, an issue arises with a consequence in which the unfired ceramic body A is liable to suffer crack due to contraction stress caused during a sintering step.
As shown in FIG. 7B, further, another attempt has been made to apply a bonding material 93 to a bonding area 95 between unfired ceramic bodies 91 and 92, which are consequently bonded to each other.
Such a method has no need to apply increased stress to the unfired ceramic body as that caused when achieving thermal compression bonding, enabling a reduction in the occurrence of deformation of the cavity portion 90.
With the ceramic bonded body obtained by applying the bonding material to the bonding area between the unfired ceramic bodies to bond these components, drying a resulting bonded body and firing the same, however, an issue arises with the occurrence of voids liable to be easily caused in the bonding material in the firing step. This results in deterioration in bonding strength of the bonding area. In such a case, further, there is a risk of causing delamination to occur at the bonding area.
In the related art, the bonding material, used for the unfired ceramic bodies to be bonded to each other, has been encountered with an increasing contraction ratio during the firing step. Therefore, it has been a general practice to adopt a bonding material having a greater contraction ratio than that of the unfired ceramic body. When using such a bonding material, the bonding area is contracted at a greater contraction ratio than those of the unfired ceramic bodies A and B during the sintering step, resulting in an issue of causing voids or delamination to easily occur as set forth above.